1. Field of the Invention
This invention relates to a method of producing a leather-like sheet material with a deluxe feeling. More particularly, it relates to a method of producing a leather-like sheet material capable of presenting such bending crimp as can give an impression of superior quality, excellent in flatness and having a high-quality feeling. The method involves, in a specific order, application of polymer layers to a substrate consisting of a fibrous base material and a porous coating layer, specific surface heat treatment, and optional embossing.
2. Description of the Prior Art
Two methods are known for providing the surface of a leather-like sheet material with an rugged pattern. One of the methods employs a supporting sheet such as a mold releasing paper or a polyethylene sheet, which has a required surface pattern, while the other comprises embossing the surface of the substrate by means of an embossing plate or an embossing roll following the preparation of the substrate. The former method is generally applied to low grade synthetic leathers, that is, leather-like sheet materials obtained by merely coating a substrate web or fabric with vinyl chloride polymer paste followed by drying. Although the reproducibility of the pattern of the support such as a mold releasing paper is good, incompleteness of the surface pattern of the supporting sheet itself renders the appearance, including the pattern and gloss of the leather-like sheet surface, evidently artificial and thus gives an impression that the resulting leather-like sheet material is a low-grade one. On the other hand, the latter method is generally used for synthetic leathers resembling natural leathers in structure in that a porous, elastic polymer layer is formed on the surface of a fibrous substrate material. By this method, the depth of the surface pattern can be varied at will by suitably adjusting the embossing conditions, and therefore a surface pattern closely resembling the grain of a natural leather can be obtained by selecting appropriate embossing conditions, said surface pattern appearing far less artificial than that produced by the former method.
The latter method, however, is not very suitable for the production of leather-like sheet materials which have such distinct and shallow grains as are present on the surface of a high-quality leather of natural origin, because, if the embossing conditions are not sufficient, the unevenness originating from that of the substrate surface, such as the unevenness due to fibers in the fibrous base material, due to the resin used as a binder for fibers, or due to pores in the porous coating layer, appears on the product surface even after the embossing. Under strong embossing conditions attained by adjusting temperature, pressure, duration and other parameters, a leather-like sheet material with vivid, shallow grains may be produced. In this case, however, another disadvantage is that the bending crimp of the sheet material, that is the crimp formed on the surface of the bent area when the sheet material is bent, appears too artificial, suggesting poor quality.
The specification of Japanese Utility Model Publication No. Sho-48-14671 discloses that, if an embossing apparatus the pressure part of which is maintained at a temperature around the softening point of the thermoplastic polymer to be used and the peeling part of which is cooled to a temperature at which the said thermoplastic polymer has a good dimension stability is used for embossing a sheet material having a surface layer consisting of a thermoplastic polymer such as polyurethane, poly(vinyl chloride) or polyamide, the sheet material can be peeled off from the embossing surface without strain on the sheet material; hence the sheet surface cannot be damaged, and consequently beautiful embossed patterns are produced. In addition, Japanese Patent Application Published under No. Sho-49-108204 discloses that a leather-like sheet material excellent in respect to bending crimp, surface luster and other properties and having a beautiful, embossed pattern is produced by applying a solution or suspension of a macromolecular substance to the surface of a porous sheet material, bringing the surface into close contact with a heated, mirror-smooth drum surface, then cooling the drum, peeling off the sheet material from the drum surface, and further embossing the sheet surface. These methods indeed improve the bending crimp and the embossing effect to a considerable extent, as described in the specifications. The bending crimp, however, is still far from that found with high-quality leathers of natural origin. Moreover, it is a great disadvantage that even and uniform embossing of the leather-like sheet surface is hard to achieve, for instance.
As a result, under the present conditions, there is a great difference in appearance between synthetic leathers provided with shallow grains by embossing, especially the ones with a mirror-smooth surface or with a pattern less than 50 microns deep, and natural leather, and the former are regarded as cheaper articles.